Shield for transformer coils



Sept. 15, 1953 c. WESTPHAL SHIELD FOR TRANSFORMER COILS Filed Aug. 22, 1949 INVENTOR. LESLIE O. WESTPHAL Patented Sept. 15, 1953 SHIELD FOR TRANSFORMER COILS Leslie C. Westphal, Voauwatosa, Wis., assignor to The Nu-Way Corporation, Rock Island, 111., a corporation of Illinois Application August 22,1949, Serial No. 111,617

(Cl. 317-206) a 6 Claims.

The present invention relates generally to transformers, with particular reference to high tension transformers of the type known as ignition transformers and used for the purpose of providing an electrical spark for igniting fuel in a fuel burning device, although the invention is not limited to this type. It is well known to those skilled in the art that when an ignition arc is produced in a fuel burner, a high frequency electrical field is produced, which, if not properly shielded, interferes with radio reception in the vicinity. Although the arc itself is shielded by the metal walls of the heater itself and the transformer is enclosed in a metal case which also performs a shielding function, this does not eliminate the interference, for high frequency waves are emanated within the transformer case from the secondary coil and are picked up by the primary winding and transmitted through the primary coil leads to the power supply circuit in the building. Thus, the wiring in the building serves as an antenna to broadcast the high frequency oscillations which interfere with radio reception.

Therefore, it is the primary object of my invention to provide shielding between the primary and secondary coils of the transformer. A more particular object has to do with the provision of a novel and efficient shielding but which is very inexpensive and is easily installed during the manufacture of the transformer.

In the accomplishment of these objects, I have provided a shield comprising a single sheet of flexible conducting material which is shaped to easily fold around the primary coil of the transformer, enclosing and shielding it from the secondary winding without interfering with its function. in the assembly. The shield is grounded to the transformer core to prevent induced voltage from building up between the shield and core.

These and other objects and advantages of my invention will be apparent to those skilled in the art after a consideration of the following de-- scription in which reference is had to the drawings appended hereto, in which 7 Figure 1 is a perspective view of a transformer,

the case of which is broken away to expose the core and coils, one of which is shielded accord-- ing to the principles of the invention.

Figure 2 is a perspective View of the transformer coil with shield installed.

Figure 3 is a plan View of the shield before it is wrapped upon the coil.

Figure 4 is an edgewise view of the shield.

Referring now to the drawings, the transformer 5 comprises a magnetic laminated core 6 of conventional design having a horizontal central leg 1 and a pair of upper and lower legs 8, 9 disposed parallel to the central leg 1 in vertically spaced relation. The adjacent ends of the horizontal legs are interconnected by vertical legs It, I! in the usual manner.

Encircling the central leg 1 are two transformer windings, a primary winding l5 and a secondary winding [6. The primary winding I5 comprises a single coil ll of wire, the two ends l8 of which are brought out for connection to the supply circuit (not shown). The secondary winding comprises two coils I9, 20, of wire, the inner ends (not shown) of which are connected together and preferably grounded on the core in a manner known to those skilled in the art and the outer ends 2|, 2?. are brought out for connection to the fuel igniter (not shown). The secondary coils are wrapped in sheets of insulating material 23, '24 to insulate them from the core. A magnetic shunt 25 is interposed between the primary and secondary windings in a well known manner.

The primary coil [1 is shielded from high frequency waves emanating from the secondary winding by means of an envelope of conducting material indicated by reference numeral 30. Although any suitable conducting material can be used which can be folded or wrapped around the coil, I prefer to use an insulating layer of cardboard or fibre 3! with a conducting layer of aluminum foil 32 adhered thereto.

The shield is cut in the shape illustrated in Figure 3, with a central portion 33 substantially coextensive with one side of the primary coil, the outline of which is indicated by the broken line C. The central portion 33 is provided with a centrally disposed rectangular aperture 34 through which the leg I of the core 6 extends when assembled. Two pairs of extensions 35,

' 36, 31, 38 project outwardly from the central portion 33. One pair of opposed extensions 35, 31 are longer than the other pair of opposed ex tensions 36, 38 and are provided with rectangular notches 39, 40 in the outer ends thereof, respectively.

The shield 30 is installed upon the coil I! by placing the latter against the insulating layer 3| at the central portion 33 in the position indicated at C, with the aperture 34 in register with the open center of the coil I1, indicated by the broken line C The two longer extensions 35, 3! are then folded over the adjacent edges of the coil 3 to form the top T and bottom B of the envelope as viewed in Figure 2. The ends of the extensions 35, 31 are then folded towards each other against the side of the coil opposite the central portion 33, and the ends are secured by adhesive tape 4| or other suitable means, in overlapping relation. The two notches 39, 40 register with each other to define a rectangular aperture in alignment with the aperture 34 to receive the core leg I.

The two shorter extensions 36, 38 can then be folded over the corresponding opposed edges of the coil and secured by tucking the corners into the enclosure formed by the extensions-33,31, as illustrated in Figure 2. Ifdesired, adhesivetape can be used to further secure the short extensions in this position. The coil ends I8 can be brought out over the edge of the extension 36 as shown.

It will now be evident that the one-pieceshield 30 completely encloses the primary coil IT to intercept any high frequency waves emanating from. the secondary windings, but does notinterfere with the leg I of the core which is assembled with the coils to extend therethrough.

A grounding wire 45 is connected with the shield 30 by a suitable rivet 46 which holds the wireli in contact with the conducting surface a 32 ofthe shield. The wire 45 can. be connected to the core 6 by a bolt 4'! after assembly to maintain the shield at groundpotential.

"A slot orgap- 48 is provided in theshield, ex'

tending from the central aperture 34 to the outer edgeof the shield, preferably at the notch 40 as shown. This gap can be cut throughthe-two layers 3 l, 32 of the sheet or merely through the conducting foil layer 32 if desired. The purpose of this gap is to prevent a circulating current from flowing around the aperture 34, which current would be induced by the alternating field in the coil I1 and by the alternating magnetic flux in the core leg T if the circuit inv theshield around the core 1 were complete. Such circulating cur .rent wouldresult in. heating of the shield and excess losses in the transformer.

The transformer'core and coil assembly is protected by a metal case 50. Preferably, the case filled with-an insulating compound (not shown) after installation of the transformer, as is well known in the art.

I do not intend my invention to be limited to the. particular details shown and described herein, except asset forth in the following claims.

I'claim:

1. In combination with a magnetic core and a coil encircling said core, a shield comprising a flexible sheet of insulating material, oneside of said sheet being covered with an electrically conductive material, said sheet including a central portion adapted to coincidewith oneside of the coil and having an aperture through whichthe core can extend,'a'pair of oppositely disposed extensions adapted to fold over opposite edge portions of said coil, and a second'pair of oppositely disposed extensions adapted to fold over other opposite edge portions and over the other side of the coil and terminating in spaced relation to eachother to receive the core therebetween, said conductive material having a gap extending from said aperture outwardly to one edge thereof.

2. A shieldedcoil for transformers and the like comprising a coil of wire having a central opening adaptedto receive a magnetic core,:a shield 4 comprising a sheet of flexible insulating material, one side of said sheet being covered with an electrically conductive material, said sheet including a central portion lying against one side of said coil and having an aperture in register with the coil opening, a pair of oppositely disposed extensions on said sheet folded over opposite edge portions of said coil, and a secondpair of oppositely disposed extensions folded over other opposite edge portions and over the other side of the coil and terminating adjacent the opening therein, said conductive material having a gap extending from said aperture outwardly to one edge thereof.

3. A. shield of the class described comprising a sheet of flexible insulating material, one side of said sheet being covered with an electrically conductive material, said sheet including a central portion having a rectangular aperture therein, four extensions extending outwardly from the foursides of said central. portion so arranged and constructed as to form an enclosure with four ends perpendicular'to'said sides of said central portion whensa-id'extensions are folded inwardly,

one opposed pairof said extensions being of sufficient length to fold inwardly parallel to said central portion in overlapping'relation' and having. notches in their outer ends adapted to form a rectangular aperture substantially in register with saidaperture in said central portion when said extensions are folded inwardly, said conductive material having av gap extending from said aperture outwardly to one edge thereof.

4. The combination set forth in claim 3, including the further provision. that saidgap in said conductive material extends from said central aperture outwardly to one of said notches.

5. In combinationwith a magnetic core and a coil encircling, said core, a shield comprising a sheet offlexible insulating. material, one side of said sheet being covered with. an electrically conductive material, said sheet including a central portion adapted to coincide with one side of the coil and having an aperture through which the core can extend, a first pair of oppositely disposed extensions adapted to fold over opposite edge portions of said coil, and a second pair of oppositely disposed extensions adapted to fold over other opposite edge portions and over the other side of the coil with the ends in overlapping relation to each other, said overlapping ends having notches therein adapted to cooperate to define a core'receiving aperture in register with the aperture in said central portion, said conductive material having a gap extending from said aperture outwardly to one edge thereof.

6. The combination set forth in claim 5, including the further provision that said gap insaid conductive material extends from said central aperture outwardly to one of said'notches.

LESLIE C. WESTPHAL.

References .Cited in the file of this patent UNITED STATES PATENTS Cooper Nov. 22, 1949 

